The invention relates generally to a method for gas turbine control and more specifically to a method for controlling the rate of transfer between operation with gas and liquid fuel types to minimize the time in undesirable operational modes, thereby preventing excessive wear and damage to gas turbine combustion hardware.
Industrial gas turbines are often capable of alternatively running on liquid and gaseous fuels, e.g., natural gas. These gas turbines have fuel supply systems for both liquid and gas fuels. The gas turbines generally do not burn both gas and liquid fuels at the same time. Rather, when the gas turbine burns liquid fuel, the gas fuel supply is turned off. Similarly, when the gas turbine burns gaseous fuel, the liquid fuel supply is turned off. Fuel transfers occur during the operation of the gas turbine as the fuel supply is switched from liquid fuel to gaseous fuel, and vice versa.
Gas turbines that burn both liquid and gaseous fuel require a liquid fuel purge system to clear the fuel nozzles in the combustors of liquid fuel. The liquid fuel supply system is generally turned off when a gas turbine operates on gaseous fuel. When the liquid fuel system is turned off, the purge system operates to flush out any remaining liquid fuel from the nozzles of the combustor and provide continuous cooling airflow to the nozzles.
FIG. 1 is a simplified schematic diagram of an exemplary gas turbine having liquid and gas fuel systems. FIG. 1 shows schematically a gas turbine 100 having liquid fuel system 102 and a liquid fuel purge system 104. The gas turbine is also capable of running on a gas, such as natural gas, and includes a gaseous fuel system 106. Other major components of the gas turbine include a main compressor 108, a combustor 110, a turbine 112 and a controller 114. The power output of the gas turbine 112 is a rotating turbine shaft 116, which may be coupled to a generator 130 that produces electric power.
In the exemplary industrial gas turbine shown, the combustor may be an annular array of combustion chambers, i.e., cans 118, each of which has a liquid fuel nozzle 120 and a gas fuel nozzle 122 (a single combustor may have one or more gas or liquid fuel nozzle depending on design). The combustor may alternatively be an annular chamber. Combustion is initiated within the combustion cans at points slightly downstream of the nozzles. Air from the compressor 108 flows around and through the combustion cans 118 to provide oxygen for combustion. Moreover, water injection nozzles 124 are arranged within the combustor 110 to add energy to the hot combustion gases.
The air for the liquid fuel system purge may be provided from the compressor 108, boosted by a purge air compressor (not shown) and controlled by other elements of the system (not shown). When the gas turbine 100 operates on natural gas (or other gaseous fuel), the liquid fuel purge system 104 blows compressed air into the liquid fuel system 102 through the liquid fuel nozzles 120 of the liquid fuel 102 system to purge liquid fuel and provide a flow of continuous cooling air to the liquid fuel nozzles 120. Similarly, when the gas turbine 100 operates on liquid fuel, the gas fuel purge system 128 blows compressed air into the gas fuel system 106 to purge gas fuel and cool the gas fuel nozzles 122.
FIG. 2 is a simplified diagram of a gas turbine engine with an existing liquid fuel system. Liquid fuel is provided to the liquid fuel system 200 from a liquid fuel source 205. The liquid fuel system 200 includes a flowpath to a flow divider 230 through a low-pressure filter 210, a fuel pump 215, a bypass control valve 220, and a stop valve 225. Pressure relief valve 235, bypass control valve 220 and stop valve 225 may recirculate liquid fuel flow through recirculation line 240 to the upstream side of the low-pressure filter 210. The flow divider 230 divides liquid fuel flow into a plurality of liquid fuel flow paths leading to individual combustion cans 270. Each liquid fuel flow path downstream of the flow divider includes a 3-way (endcover) valve 245 and a distribution valve 260 before entering the combustion can 270.
Three-way valve 245 permits flow to the combustion can nozzles from the liquid fuel flow path (described above) or from a liquid fuel purge air system 280. Three-way valve 245 is designed to selectably allow flow to the combustor nozzles 120 from the liquid fuel while preventing backflow of fuel to the liquid fuel purge air system or to allow purge air to the combustor nozzles 120 while preventing backflow of purge air into the liquid fuel system upstream of the three-way valve. By preventing purge air from entering the liquid fuel system, the air-fuel interfaces with the fuel supply are minimized.
When gas fuel is supplying the turbine, the 3-way valve 245 is positioned to block liquid fuel flow and allow purge air to pass for cooling the fuel nozzles in the combustor. This purge must be shut off when liquid fuel is turned on. The 3-way (endcover) isolation valve separate the purge air from the liquid fuel. The fuel system does not require this hardware; the use of 2 check valves, or 2 way on/off valves could be employed.
FIG. 3 illustrates a simplified gas fuel system. The gas fuel system 300 includes a gas fuel source 305 and a purge air source 340. The gas fuel system further includes a gas stop valve 310, a gas vent valve 315, and gas control valves 320. Gas control valves 320 controls the amount of gas fuel admitted to manifolds 325 and the associated gas nozzles 330 downstream of the manifolds 325.
The purge air source 340 provides compressed air to the combustor gas nozzles to purge the piping and nozzles of gas when using the gas fuel and to cool the gas nozzles when the gas fuel is no longer supplying the gas turbine. Each line to an individual manifold 325 includes blocking valves 345 and 350 (to satisfy leakage requirements and ensure air and fuel do not mix), vent valve 355, and an orifice 360 for limiting purge air flow.
Several problems may arise in a gas turbine during low fuel flows. When on liquid fuel and transferring to gas, the turbine experiences low gas fuel flow for a certain period of time while gas fuel is being ramped on, and liquid fuel ramped off. At the end of this same transfer, low liquid fuel flow is seen at the end of the transfer when the unit is nearly on 100% gas fuel. Similarly, on gas fuel transferring to liquid fuel there are periods of low flow operation at the beginning of the transfer when liquid begins to come on, and at the end of the transfer when gas is nearly off.
When on gas fuel and operating at low fuel flows, inaccuracies in gas control valve(s) allow for the potential of incorrect fuel split scheduling. This incorrect gas fuel split can cause high combustion dynamics, which leads to potential combustion hardware damage and increased wear. As gas fuel flow drops the nozzle pressure ratio drops. The nozzles are designed for a minimum steady state pressure ratio (1.025 typically), and this is done to avoid a dynamic coupling between the fuel system and combustor. During fuel transfers this minimum ratio is temporarily violated. Where that violation occurs is a function of the load on the turbine at the time of the transfer and the number of gas fuel circuits in operation during the transfer. This is a source of combustion dynamics leading to potential damage and gas fuel nozzle wear.
Similar equipment wear and potential damage is possible on the liquid fuel nozzle at low liquid fuel flows. The typical gas turbine uses a can annular system where the combustion cans are located at different elevations. Currently a mechanical flow divider (connected positive displacement pumps) is used in the liquid fuel system to compensate for this head difference, mostly for the very low flow condition during the startup of the unit. A single can within the liquid fuel system may consist of numerous cartridges (nozzles). Due to elevation differences between the cartridges and head effects of the liquid fuel within the can, the highest cartridge will lose liquid fuel flow first.
FIG. 4 illustrates differences in head driving liquid fuel source 370 to flow through liquid fuel nozzles within a combustor can be dependent on nozzle positions. A side elevation for two simplified combustor cans 410 and 450 are shown, where combustor can 410 is higher than combustor can 450 by height H1. Combustor can 410 has two nozzles 415 and 420 separated by height H2. Combustor can 450 has two nozzles 455 and 460 also at different elevations. The flow divider 230 ensures an appropriate distribution of liquid fuel flow to each of the individual cans. However, the nozzles within an individual cans are at different elevations. The lower nozzles therefore have different flows based on the different head due to elevation. There is no mechanism to evenly divide the flow to all nozzles at different elevations. The highest cartridges with the least flow head behind the fuel receives the least fuel flow. These highest cartridges then experience increased wear and potential damage between the period of time when liquid fuel stops flowing through them, and liquid fuel purge is initiated. Purge flow cannot be turned on until liquid fuel has stopped flowing as there is a significant risk of introducing liquid fuel into the purge system.
Both high combustion dynamics due to inaccurate gas fuel splits/low pressure ratio across the fuel nozzles and the potential for damage/increased wear due to low liquid fuel flows prior to purge initiation are problems corrected by the present inventive method.
Historically, the transfer between gas and liquid fuel operation has been controlled with a constant, selectable, ramp rate that reduces one fuel and increases the other correspondingly. During a liquid to gas fuel transfer, the gas fuel button is selected from the operator screen. Once all conditions for fuel transfer are met and a gas prefill is completed, the liquid fuel is slowly ramped off at a constant rate until liquid fuel flow is reduced to 0%. At the same time gas fuel flow is increased until reaching 100%. Halfway through the transfer, the turbine will be running on 50% liquid and 50% gas fuel. The basic building blocks of the fuel transfers are used for operation of “Mixed Fuel” where both fuels are held at some split, as part of steady state operation. There has always been a minimum allowable steady state split between the fuels to avoid some of the issues described herein during the transient transfer.
At low gas fuel flow levels, gas control valve accuracy can vary widely from expected. A requested position of 5% stroke might be expected to flow 5% of valve capacity (linear relationship), but instead the valve may be flowing 5%+/−3% of its capacity. This gas fuel flow error results because the gas control valve is not typically calibrated below 10% stroke, as the turbine is not intended to run with valves in these low stroke conditions for any length of time. The inaccuracy at low gas control valve strokes will cause incorrect fuel splits to be sent into the combustor and lead to high dynamics within the combustion system. High dynamics are known to increase combustion system wear and decrease part life.
At low liquid fuel flows field-testing has shown there is not enough head pressure to provide sufficient liquid flow to all liquid fuel nozzle cartridges. Increased fuel nozzle wear and potential damage occurs while on low liquid fuel flow prior to initiation of purge airflow.
During a transfer from one fuel source to another, it is desired that continuity of turbine output power be maintained while minimizing any undershoots or overshoots of output power and temperature. In a transfer from operation with a gas fuel to operation with a liquid fuel, the 3-way valve 245 is switched to the liquid fuel line, the stop valve is opened, and the control valve is commanded to some small prefill flow. As the liquid fuel line is prefilled, the gas fuel is held at a required demand reference for the generator output output. The liquid fuel must refill the piping, which was previously filled with purge air, before liquid fuel reaches the combustor nozzles 120.
Controlled fuel is fuel that is provided to the combustors in response to a fuel reference demand for a given power output. Uncontrolled fuel is fuel that is introduced to the combustors, but which is not recognized in the turbine control fuel demand signals. Liquid fuel prefill is not included in the fuel demand calculation to avoid a detrimental dip in load if the liquid fuel prefill does not reach the combustors as expected, thus creating an under-fueled condition. The side effects of the uncontrolled fuel reaching the combustors is to supply additional energy resulting in an initial load and temperature spike and then causing global fuel demand to be driven down over time to hold the same load output.
Prefill rates may be limited by operational procedures at specific gas turbines, which may be administratively changed. However, high prefill rates may also affect the initial load and temperature spikes described above. Therefore, it may not be possible to implement higher prefill flow rates without incurring penalties in the form or undesirable or unacceptable load and temperature transients.
FIG. 5 illustrates a prior art algorithm for control of a transfer between fuel types. For exemplary purposes the fuel types are described as fuel “A” and fuel “B”. The algorithm provides for a transfer from fuel “A” to fuel “B”. Initially in step 510, the fuel type to which the load will be transferred is selected and designated as fuel “A”. In step 520, a fuel prefill is completed with fuel “A”. In step 530, it is determined whether the load is currently being powered by gas fuel, meaning that the load is being switched to liquid fuel (fuel “A” is liquid fuel). If fuel “A” is liquid fuel, then in step 540 a constant first fuel transfer ramp rate is selected. If the load is not being switched to the liquid fuel (but to gas fuel), then in step 550 a constant second fuel transfer rate is selected. The first fuel transfer ramp rate may be higher (usually twice as high) than the second fuel transfer ramp rate to recognize that the slow fuel supply is gas and that a transfer to a liquid fuel may be an emergency shift which requires a faster transfer to prevent loss of load. In step 560 the percent fuel “A” is increased at the fuel transfer ramp rate and the percent fuel “B” is decreased at the fuel transfer ramp rate. In step 570, a check is performed to determine whether operation with 100% fuel “A” has been achieved. If 100% fuel “A” operation has been achieved, then the transfer is complete in step 580. If 100% operation with fuel “A” has not been achieved then step 560 is continued until full fuel “A” operation has been achieved.
FIG. 6 illustrates simplified fuel transfer rates for fuel transfers from fuel “A” to fuel “B”, and for transfers from fuel “B” to fuel “A” under the prior art method. The vertical axis represents the percent of a fuel type that is being supplied during two fuel transfer operations. The horizontal axis represents an unsealed time axis showing the relative transfer times during the two fuel transfer operations. The solid curve 610 represents fuel type “A”. The dashed line 620 represents fuel type “B”. For fuel transfer from fuel “A” to fuel “B”, the slope (transfer rate) is twice at high as the slope (transfer rate) from fuel “B” to fuel “A”. Consequently, the transfer time 630 for a fuel transfer from fuel “B” to fuel “A” is twice that of the reverse transfer 640. The fuel transfer rates are linear, reflecting a constant transfer rate throughout the process. FIG. 6 also illustrates a prefill process 650 for the transfers. Prefilling is performed by supplying the fuel which is to pick up the load earlier than demanded by turbine load control signals in order to ensure that the fuel supply lines for the oncoming fuel source are purged of air and filled with fuel when required to supply turbine load.
FIG. 7 illustrates high nozzle temperatures resulting from low liquid fuel flow during fuel transfers under the prior art method. The left vertical axis indicates values for megawatt output 710, liquid fuel stroke reference 720, gas fuel stroke reference 730 and percent liquid fuel 740. The right vertical axis indicates values for nozzle temperatures 750, 760, 770 at various locations around the turbine. As fuel is transferred from liquid fuel to a gas fuel, the percent liquid fuel 740 decreases at a constant ramp rate and the gas fuel stroke reference 720 ramps up reflective of gas fuel flow to the combustor. When the percent liquid fuel 740 drops below about 10%, nozzle temperatures 750, 760, 770 begin to rise, continuing to increase as percent liquid fuel 740 drops to 0%. Nozzle temperature 750, 760, 770 finally drop 780 as a result of liquid fuel purge coming on.
Accordingly, there is a need to conduct fuel transfers in a manner, which increases reliability and decreases the potential for hardware damage and wear, thereby prolonging life of the equipment. Also, there is a need to perform the modified transfers without the need for hardware changes and the associated cost and time for such modifications.